Although steam engines can use a variety of liquid and lower cost solid fuels and inherently produce few toxic emissions, the thermal efficiency of current steam engines is significantly below that of internal combustion engines.
To achieve greater efficiency, Applicant's prior application Ser. No. 12/959,025, filed Dec. 2, 2010, now U.S. Pat. No. 8,448,440 provides a new Rankine operating cycle which can be referred to as a “zero clearance with zero compression” cycle or a Z-Z cycle in which efficiency was improved in part by arranging the clearance between the piston and cylinder head to approach zero but provision must be made to keep the piston from hitting the head. Consequently, a near zero or quasi-zero clearance was used. While minimal compression is typically provided in both the present and the prior application, for various reasons some compression may be present during the last fraction of an inch before TDC due, for example, to the dimensions of the valves and parts used to operate valves.
U.S. Pat. No. 4,168,655 describes an engine with an automatic inlet valve in which the force used to close valve is overcome by knocking it ballistically off its seat. However, this produces a high impact stresses in the valve and piston. In addition to eliminating the high impact stress of U.S. Pat. No. 4,168,655, the piston of the present invention can be connected directly to a wrist pin and connecting rod if desired and does not need an external valve structure that adds substantially to the clearance volume. The prior patent also employs steam recompression which makes zero compression with zero clearance operation impossible. In addition routine variations in speed and pressure would interfere with an attempt to provide repeatable cutoff control. It is also current practice, for example as in U.S. Pat. Nos. 7,856,822 and 7,992,386 to admit steam through inlet passages or ports that extend outwardly from the cylinder and therefore create a substantial clearance volume.